The present application relates to electrically superconducting films and to the production of such films, and more particularly to the production of thin films of Perovskite-type superconducting oxides, commonly referred to in the art as 1-2-3 compounds.
Recently it has been found that a remarkable improvement of the critical current densities of Perovskites is obtained if epitaxial cuprate Perovskites, such as, Y Ba.sub.2 Cu.sub.3 O.sub.7, are utilized in the form of thin, orientated films.
It is presently known that cuprate Perovskites may be prepared in the form of thin films by deposition on heated substrates, typically temperatures in the range of 400 to 800 degrees C. are employed. The deposition process is carried under vacuum and an initial thin amorphous film, desireably comprised of oxides containing the metallic elements present in a weight ratio of 1:2:3 based on the atomic mass of Y, Ba and Cu, is deposited on the substrate. Suitably the film may be deposited on the substrate by sputtering using a magnetron, or by evaporation using an excimer laser. The Perovskite materials may be supplied as separate source materials, or targets, or they may be supplied as a single composite source, or target. The structure of the initially formed film is converted into Perovskite-like structure by subsequent treatment steps. For example, the film is typically first heat treated, or annealed, over a period of time at temperatures ranging between about 700 and about 900 degrees C. and later the oxygen content and the desired structure of the heat treated film is adjusted to the desired level by a further heat treatment step, typically carried out at temperatures in the range of between about 500 to about 600 degrees C., in an oxygen atmosphere.
Various proposals have been made to combine the described processing steps. For example, it has recently been proposed to initially carry out the deposition step at a higher substrate temperature, typically from about 650 to about 750 degrees C., in an oxygen atmosphere in an attempt to adjust the oxygen content of the film during the deposition step and in this manner eliminate the intermediate heat treating step.
In any case, the high temperatures, over 500 degrees C., that are employed by the prior art processes severely limit the materials that can be utilized as substrates and cause interdiffusion of the film and the substrate surface. Usually a sharp interface, between the film and substrate or between the film and a buffer layer, is highly desirable and frequently required. The diffusion problem can not simply be solved by the use of lower processing temperatures because there is a minimum temperature, generally about 400 degrees C., that is required to insure that the film will adhere to the substrate. Further, it is known in the art that cuprate Perovskites, for example, Y Ba.sub.2 Cu.sub.3 O.sub.7, at temperatures below about 450 C. have an orthorhombic crystalline structure and are conducive to superconductivity, while at temperatures above about 450 degrees C. such Perovskites begin to loose their superconductive properties. Thus, a high temperature treatment step must always be followed by a low temperature treatment step in order to obtain the desired orthorhombic structure.